The study is a randomized controlled trial with the intervention being the introduction of either pureed/strained beef or iron-fortified cereal as the first complementary food of breastfed infants at 4-5 months of age. It is hypothesized that the infants fed beef will have greater growth velocity and a more positive mean slope in the delta Z- score for each of the growth indices between 4-7 mos & 4-12 mo; significantly better zinc & iron status at 7 mos, and; significantly higher scores on the Bayley Scales of Infant Development.